Complex Segmental & Epicyclic Gears
SEGMENTAL GEARS ("a piece for receiving or communicating reciprocating motion from or to a cogwheel, consisting of a sector of a circular gear, or ring, having cogs on the periphery, or face." and epicyclic gears were both first invented by the 11th century Arab engineer Ibn Khalaf al-Muradi from Islamic Spain. He employed both these types of gears in the gear trains of his mechanical clocks. Simple gears have been known before him, but this was the first known case of complex gears used to transmit high torque.
Segmental gears were also later employed by al-Jazari in 1206. According to Professor Lynn Townsend White, Jr. "Segmental gears first clearly appear in Al-Jazari, in the West they emerge in Giovanni de Dondi‘s astronomical clock finished in 1364, and only with the great Sienese engineer Francesco di Giorgio (1501) did they enter the general vocabulary of European machine design."
Today Epicyclic gearing or planetary gearing is a gear system consisting of one or more outer gears, or planet gears, revolving about a central, or sun gear. Typically, the planet gears are mounted on a movable arm or carrier which itself may rotate relative to the sun gear. Epicyclic gearing systems also incorporate the use of an outer ring gear or annulus, which meshes with the planet gears.
In normal operation epicyclic gear designs the planet pins are straddle mounted on a rigid carrier and are precisely aligned to each other. If they are not the load distribution across the face is affected, but not the load sharing. The sun pinion and flexible annulus are centered by the planet wheels when under load.The axes of all gears are usually parallel, but for special cases like pencil sharpeners they can be placed at an angle, introducing elements of bevel gear. Further, the sun, planet carrier and annulus axes are usually concentric.
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